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This paper reports on the fabrication and test of a hydrogel-actuated microvalve that responds to changes in the concentration of specific chemical species in an external liquid environment. The microvalve consists of a thin hydrogel, sandwiched between a stiff porous membrane and a flexible silicone rubber diaphragm. Swelling and deswelling of the hydrogel, which results from the diffusion of chemical species through the porous membrane is accompanied by the deflection of the diaphragm and hence closure and opening of the valve intake orifice. A phenylboronic acid based hydrogel was used to construct a smart microvalve that responds to the changes in the glucose and pH concentrations. The fastest response time (for a pH concentration cycle) achieved was 7 minutes using a 30 µm thick hydrogel and a 60 µm thick porous membrane with 0.1 µm pore size and 40% porosity.

We review efforts to produce microfabricated glucose sensors and closed loop insulin delivery systems. These devices function due to the swelling and shrinking of glucose-sensitive microgels that are incorporated into silicon-based microdevices. The glucose response of the hydrogel is due to incorporated phenylboronic acid (PBA) side chains. It is shown that in the presence of glucose, these polymers alter their swelling properties, either by ionization or by formation of glucose-mediated reversible crosslinks. Swelling pressures impinge on microdevice structures, leading either to a change in resonant frequency of a microcircuit, or valving action. Potential areas for future development and improvement are described. Finally, an asymmetric nano-microporous membrane, which may be integrated with the glucose sensitive devices, is described. This membrane, formed using photolithography and block polymer assembly techniques, can be functionalized to enhance its biocompatibility and solute size selectivity. The work described here features the interplay of design considerations at the supramolecular, nano, and micro scales.

In this paper, we report on the design and characterization of a new hydrogel-based implantable wireless glucose sensor. The basic device structure is a passive [inductor/capacitor (LC)] micromachined resonator coupled to a stimuli-sensitive hydrogel, which is confined between a stiff nanoporous membrane and a thin glass diaphragm. As glucose molecules pass through the nanoporous membrane, the hydrogel swells and deflects the flexible glass diaphragm, which is the movable plate of the variable capacitor in the totally integrated passive LC resonator. The corresponding change in resonant frequency can be remotely detected. A glucose- sensitive phenylboronic acid-based hydrogel was loaded into the microtransponder, and its sensitivity and time response were measured. Prior to hydrogel loading, the sensitivity of the pressure sensor to applied air pressure was measured to be -222 kHz/kPa over the frequency range 51-->42 MHz. The sensor showed a sensitivity of -34.3 kHz/mM over the glucose concentration range 0-20 mM (at pH 7.4), and a response time of 90 min. The dynamic response, although unacceptable at such values, can be easily improved by decreasing the hydrogel thickness and reducing the sensor and porous membrane thicknesses. The transponder's overall dimensions were 5x5x0.8 mm3, small enough for subcutaneous implantation.

The use of microsensors for in-field monitoring of environmental parameters is gaining interest due to their advantages over conventional sensors. Among them microsensors based on semiconductor technology offer additional advantages such as small size, robustness, low output impedance and rapid response. Besides, the technology used allows integration of circuitry and multiple sensors in the same substrate and accordingly they can be implemented in compact probes for particular applications e.g., in situ monitoring and/or on-line measurements. In the field of microsensors for environmental applications, Ion Selective Field Effect Transistors (ISFETs) have a special interest. They are particularly helpful for measuring pH and other ions in small volumes and they can be integrated in compact flow cells for continuous measurements. In this paper the technologies used to fabricate ISFETs and a review of the role of ISFETs in the environmental field are presented.

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In healthcare, new diagnostic tools that help in the diagnosis, prognosis, and monitoring of diseases rapidly and accurately are in high demand. For in-situ measurement of disease or infection biomarkers, point-of-care devices provide a dramatic speed advantage over conventional techniques, thus aiding clinicians in decision-making. During the last decade, paper-based analytical devices, combining paper substrates and electrochemical detection components, have emerged as important point-of-need diagnostic tools. This review highlights significant works on this topic over the last five years, from 2015 to 2019. The most relevant articles published in 2018 and 2019 are examined in detail, focusing on device fabrication techniques and materials applied to the production of paper fluidic and electrochemical cell architectures as well as on the final device assembly. Two main approaches were identified, that are, on one hand, those ones where the fabrication of the electrochemical cell is done on the paper substrate, where the fluidic structures are also defined, and, on the other hand, the fabrication of those ones where the electrochemical cell and liquid-driving paper component are defined on different substrates and then heterogeneously assembled. The main limitations of the current technologies are outlined and an outlook on the current technology status and future prospects is given.

The suitability of polysilicon as a material for the fabrication of interdigitated electrodes and their application to the development of sensors is studied in this work. The main interest in using this material lies in the possibility of obtaining integrated sensors with commercial CMOS technologies and simple post-processing steps. Electrodes with 3μm finger width and 3, 10, and 20μm spacing were fabricated and characterised. Conductivity measurements in the range from 4 to 50μS/cm yielded a linear response with cell constants of 0.04cm prostate-specific antigen (PSA) [11], bacteria [12], red blood cells (hematocrit) [13], and detection of specific sequences of DNA [14]. In some applications, the measurement of a physical magnitude such as dielectric constant or conductivity of the media provides direct information of the analyte concentration [5,13]. In other applications, the deposition of a membrane or functional monolayer on top of the electrodes as a recognition element is necessary to obtain sensitivity and selectivity to particular species [1][2][3][4][6][7][8][9][10][11][12][13][14]. Different arrangements of electrodes (i.e. different number of electrodes and geometries) may be used for impedimetric measurements. Among them, interdigitated electrodes have significant advantages for certain applications: 1) their low cell constant (resistance to resistivity ratio) permit the measurement of very low conductivity solutions, 2) measurement of dielectric properties is possible thanks to a high inter-electrode capacitance (higher than stray capacitance even for conductive substrates), 3) the short penetration depth 3 of electric fields make them less dependent on the measurement cell geometry and allow the use of thin membranes for tailoring selectivity.In this paper the suitability of polysilicon as a material for the fabrication of interdigitated electrodes and their application to the development of sensors is studied. For example, the feasibility of measuring solution conductivity and permittivity with polysilicon electrodes is demonstrated. The main interest in using this material lies in the possibility of fabricating the electrodes with commercial CMOS technologies and simple post-processing steps, and therefore being able to integrate them as sensors in a more complex system-onchip. Another interesting advantage is that this material can be easily modified following a one-step silanisation reaction. In this way, robust immobilisation of either biomolecules or membranes can be carried out on to both the electrode fingers and the area in between them with the aim of developing a sensor device. As an example, a fully functionalised biosensor for the detection of urea is also presented.The basic structure of the interdigitated electrodes is depicted in Fig. 1. The polysilicon traces forming the interdigitated geometry are separated from the silicon substrate by a thick silicon dioxide layer. represented by R sol and C so l . From these components, the resistivity ρ (or the conductivity σ) and th...